Method of chromium plating



. which have been commonly. employed in the Patented May 16, 1933 UNITED STATES PATENT OFFICE WALTER L. rm 0! DETROIT, IIOHIGAN, 88161703. '.I'. O GENERAL SPRING BUMPER OORPOBL'I'IOI, OI DMROI'I', IIOHIGAIN, COMORLTION OF MICHIGAN amnion or enamel: ru'rme Io Drawing.

This invention relates to a method of chromium lating, and more particularly to a metho of chromium lating whereby a coating of electrodeposi chromium relatively free from pores or cracks .15 obtained. The coating secured, therefore, 1s far more suitable for use in protecting a base metal from atmos heric or other corrosion than is the case 0 de sits produced by the processes that have b en heretofore known, and

art.

My invention, therefore, has as its object the production of more impervious deposits of electrodeposited chromium and the improvement 1n the art of protecting base metals from corrosion by electrolytically deposited chromium.

For the electrodeposition of chromium I employ a solution consisting principally of chromic acid and an adde substance such as sulphuric acid or a sulphate or another salt or acid, these being employed in such proportion as will give a. satisfactory deposlt of chromium. An suitable chromic acid chromium lating bath may, therefore, be employed, but I prefer to use one of the chromium plating solutions such as described by G. J. Sargent, and commonly known as the Sargent solution. This solution would contain, for example, 250 grams of chromic acid (CrO per liter of solution and 2.5 grams of sulphuric acid (H SO4) er liter of solution. A suitable anode, for instance lead, is employed. The cathode current density which--is suitable depends on the composition of the bath, and may also vary with the temperature, as discussed in the prior art, and maybe, for example, a cathode current densit of 100amperes per square foot when the bath is operated at a temperature of 115 F. If the bath temperature is increased, the current densit would be increased, and likewise if the bat temperature is decreased, the current density would be decreased.

It has been well established in the prior art that when chromium is electrolytically deposited as a bright or lustrious deposit from any suitable chromium plating solution that manu... ma llovember 1:, 1930. Serial Io. 495,428.

has as its major constituent chromic acid, the porosity or imperviousness changes with increasing thickness of deposit. The first .chromium deposited fails to completely trodeposited chromium differs from other commonly electrodeposited metals, however, in that when the thicknessof metal exceeds the order of 0.00002 inches, this stress results in the formation of fine cracks which extend through the electrodeposited chromium, and results therefore, in a porous condition. As the thickness of the electrodeposited chromium is still further increased, the number of these fine cracks is decreased, and in their place there is a smaller number of much arger cracks. The net effect, therefore, after this action starts is a progressive increase of porosity and a pro ressive decrease in the protective value 0 such deposits, which continues until very great thicknesses .of deposits relatively, are obtained, these thicknesses bein prohibitive from the standpoint of manu acturing cost in any commercially operable process.

There is, therefore, an optimum thickness of deposits'which gives the least porosity, but this porosity is by no means zero, andthe electrodeposited chromium, therefore, byany process which has heretofore been known, fails to give to the base metal the protection which it is desired to create by the electrodeposited chromium.

The precise thickness at which minimum porosity, as discussed above, would be obtained varies somewhat with the composition of the bath employed, and the-conditions used for electrolytically depositing the chromium. All of this has been well discussed in the prior art, notabl in the following publications: Protective Value of Chromium Plate by E. M. Baker and W. L. Pinner, S. A. E. Journal, March 1928; Porosity of Electrodeposited Chromium by E. M. Baker and A. M. Renta, The.

Transactions of the American Electrochemical Society, Vol. LIV, 1928; and Notes on the Hardness and Structure of Deposited Chromium by Leland E. Grant and Leroy F. Grant, The Transactions of the American Electrochemical Society, Vol. LIII, 1928.

It is to be noted that this porous character of electrodeposited chromium has been well recognized as has also been the desirability of producing a deposit which is more nearly impervious. Except for my invention, all processes which have been at all successful along this line have involved heating of the deposit to remove the hydrogen which is adsorbed or occluded within the electrodeposited chromium. Such a process is described by R. J. VVirshing in Heat Treatment of Chromium Deposits to Increase Their Resistance to Corrosion, The Transactions of the American Electrochemical Society, Vol. LVIII', 1930. This process, however, of heating the deposit and, of course, with it the metal on which the deposit is made, is expensive.

I have now discovered a method by which I can achieve progressively decreasing porosity of a chromium deposit with increasing thickness, and at substantially no greater cost than is required to produce the chromium deposits heretofore known in the art. My process consists simply in dividing the total plating time into two or more increments. For example, if by the old process, the plating were to be done at a current den sity of 100 amperes per square foot and for a time of four minutes, by my process I would plate, for example, at this current density for 2 minutes then stop the plating. The exact duration of time for which the plating is stopped is not critical, and may, for example, be from 1 second to a much longer period of time, as will appear in the further description of my process. The plating would then be resumed for a second period of 2 minutes. Expressed diflerently, my process contemplates, in a specific case, chromium plating until 200 ampere minutes of electricity per square foot of cathode surface have passed through the plating circuit, breaking the circuit and again plating until a like quantity of electricity has passed through the circuit.

While this process is simple, it produces a tremendous and wholly unexpected result With respect to the porosity of the deposit obtained. For example, if steel, which has protective value in the standard salt spray test might be 35 hours before the appearance of rust. When the chromium deposit, how-- ever, is applied in two increments by my process, as described above, a protective value of from 150 to 200 hours in the salt spray can be obtained.

I nave achieved even superior results by dividing the total deposit into 3 increments, as in the above case for example, of 1% minutes each. When thicker deposits are to be made, as for instance, if the total plating time, as described above were 10 minutes, then better results would be obtained by using more than 3 increments for the deposition of the chromium. In fact whereas it is well established in the prior art that steel cannot be adequately protected from atmospheric corrosion by the electrodeposition of chromium alone, I find that I can produce on low carbon steel a protective value, as measured in the salt spray, of 40 hours before the appearance of rust by plating with chromium for a total time of 7 minutes when this is divided into 5 increments of 1.5 minutes each.

I have stated that in byv process the deposition of chromium is interrupted one or more times in the course of forming the total deposit. Such interruption of deposition is obtained, for instance, by merely lowering the current density to a point at which chromium ceases to be deposited. It is notnecessary that the current be totally stopped from flowing, but merely that the current be reduced to a value where chromium is no longer deposited and where the actions at the cathode consist of such things as the deposition of hydrogen and the reduction of .6. valent chromium to lower states of oxidation.

I prefer, however, when this maybe conveniently done, to discontinue entirely the flow of current in the intervals between the several increments of the plating. In fact I achieve better results from the standpoint of porosity and protective value if I remove the articles from the bath and rinse them in water and then return them to the bath for the next increment of chromium plating. Even better results are obtained if after rinsing with water, the articles are rinsed in dilute acid, for instance, a 5% solution of hydrochloric acid or sulphuric acid and are then again rinsed in water before they are returned to the chromium plating bath for the next increment of chromium plating. It will be obvious, however, that these procedures, while increasing the protective value,

' from the chromiumplating bathbetweenthe for the labor of o eration and also for the chromic acid whic is lost or washed away when the articles are removed from the chromium plating bath and rinsed. Likewise where this procedure is employed, there would be the expense of acid for the acid dip referred to.

If it is necessary that the articles be coated with a bright or lustrous deposit of chromium at the completion of the, process, it is best to observe certain precautions in the operation of this process. When a subsequent increment of chromium is to be deposited, the current should not be increased to the full valueimmediately, but rather the'voltage be-. tween the anode and cathode should-be in-' creased froma low value at a gradualrate. This minimizes the likelihood of a grey or cloudy, second or subsequent deposit of chromium-being obtained. Also if the artic185 being chromium plated are removed severall'incremen'ts of plating, it is best to leave them in the. chromium..plating bath .for a. few'seconds so that the articles may assume the temperature of the bath, and so that the water in which they were last rinsed.

. may'-be replaced,.by the-process at diffusion,

by .thechromium'plating solution before the current'is again turned on for. thenext iii-- crement of plating. I also find that it aids in maintaining the brightness of the" deposit. to use a current density for the subsequent increments somewhat lower than the current density used for the first increment. 'The current density used with the subsequent increments may be 85% of thecurrent den' sity used for the first increment. I do not wish to imply, however, that either the slow reestablishment of current density or the reduction or change of the current density between increments is essential to my process. While the same current density may be rapidly reestablished and applied for all increments, I find slightly improved results as regards luster with the modified procedure just described.

When the chromium plating operation is completed, as usual in the art, I rinse the plated articles first with cold water, then with hot water. The articles may be used in this condition. However, I find that the luster of the articles is improved and that some further improvement in qualityfrom the standpoint of resistance to corrosion is obtained if the chromium plated articles are buffed. This operation of bufling chromium plated articles is not in itself new, as bufling has been employed throughout the art when a grey or milky deposit inadvertently re 'sulted from the chromium plating operation.

Buffing of the chromium deposit for the express purpose of improving the rust resistance or protective quality as contrasted to bufling to remove dull or milky plating is to the best of my knowledge new. This bufiing, while desirable, is not an end in itself nor of the essence of my invention.

the purpose of. gettingsome chromium-on all. e.

surfaces of the artic "Some arts oft-he article plated according to the o d operation would vhave,therefore," only a single deposit.

.-My processis directed to the securing of two or more deposits on all of the those surfaces.-

of -the;. article.-whichwit"is desired to protect by the .electrodeposit L @chmmium "Fury-h thermore,.. it should be" noted that in my;

process eachplating"increment,fincluding; the first constitutes an 'electrodeposit of; chromium that is a commerciall-y'satlsfactory one, ex ept that it. is not sufiiciently imj pervious according to my standards and possibly not sufficiently thick for some purposes. In" my process, thefirstplating in-H crement. deposits chromium on all those surfaces which it is desired to protect, and is a deposit such as heretofore would not, under good commercial practice, be given a second plating. My process is, therefore, tobe distinguished from any case of double plating due to adefective first deposit. As used in the claims, the term commercially satisfactory, as applied to the chromium deposit, means a deposit free from such defects as would, in common commercial practice heretofore, have required a second platmg.

Although I have described for the pur ose of illustration, certain specific ways in w ich my invention may be carried out, it is to be understood that within the scope of my invention, wide deviations may be made therefrom without departing from the spirit of my invention. 1

I claim as my invention:

1. The process of chromium plating as a continuous treatment, which comprises electro-depositing a layer of chromium of a bright character'on an articleto be plated, reducing the current to a point at which electro-deposition of chromium ceases without removal of the article from the bath, and again passing a current of plating strength through the bath to deposit a second coating of chromium also of a bright character on the first coating.

2. The process of chromium plating as a continuous treatment which comprises passing a current through a bath until a bright County,

but relatively thin and porous layer of chromium has been deposited upon an articleto be plated, reducing the current below plating strength and subsequently passing a current of plating strength through the same bath to deposit a second layer of chromium of a bright character on said first layer, the combined layers having considerably reduced porosity as compared with that of said first layer or with that of a layer of the same thickness obtained in a single continuous plating step under otherwise identical conditions of plating.

In testimony whereof I have hereunto subscribed my name at Detroit, Wayne Michigan.

WALTER L. PINNER. 

